This invention relates to methods and apparatuses for quickly measuring response and distortion characteristics of a signal transfer device, and in particular, for measuring the signal transfer characteristics of an audio signal transfer device, such as an audio amplifier, a telephone channel, or the like.
In the sound reproduction, broadcast and telephone communications industries it is often necessary or desirable to determine the response and distortion characteristics of a signal transfer device in order to evaluate, repair or improve an audio signal transfer path. The transfer device may be any of many commonly known devices, such as a stereo sound reproduction amplifier (sometimes referred to as a "hi-fi" or "stereo" amplifier), a telephone communications channel or other communications link, a magnetic tape audio signal recorder, or audio signal broadcast equipment. What these devices have in common is that they accept as an input one or more audio frequency signals and reproduce them at their output, either immediately or delayed in time.
The conventional way of measuring response and distortion characteristics of such devices ordinarily has been to apply sequentially one or more known test tones, i.e., audio frequency sinewave signals whose frequencies are known, to the input of an audio signal transfer device and to measure their amplitude and relative phase, and the amplitude of their harmonics, at the output of the device. The tones are usually measured at the output using a bandpass filter whose center frequency is tuned to one of the input frequencies or a harmonic thereof. For example, to determine the frequency response of such a device, a number of tones are applied sequentially to the input and their amplitudes are measured at the output to obtain data representative of output amplitude as a function of frequency. Similarly, to measure harmonic distortion a known frequency is applied to the input of a device and the amplitudes of those harmonics of the input frequency which are present at the output are measured as an indication of harmonic distortion. Conventional testing with manually-operated equipment can take as much as an hour for a thorough evaluation. This can be reduced with computer-controlled equipment, but still requires a significant amount of time during which the device under test (hereinafter "DUT") cannot be used for normal activity. It is to be understood that the terms device or DUT used herein refer without limitation to one or more devices connected together.
A recognized international measurement standard has been adopted by the CCITT (International Consultative Committee on Telephone and Telegraph) and the EBU (the European Broadcasting Union) which employs two sequences of test tones, one for monophonic devices and one for stereo devices. The monophonic sequence lasts 31 seconds and the stereophonic sequence lasts 33 seconds. The sequence begins with a preamble which indicates which sequence is being sent and the originator of the test. The tones are one second in duration each, and an 8 second long pause is included for noise measurements. However, while this method has the advantage of being standardized, like other conventional test methods it is relatively time consuming; that is, it requires the device under test to be shut down from normal activity for a significant amount of time, thereby disrupting normal operations.
There are also devices known as real time analyzers that apply a white noise signal to the input of a DUT and provide an indication of the output amplitude at a number of frequencies by the simultaneous use of multiple bandpass filters tuned to those frequencies. The amplitudes of the outputs from those filters are displayed. However, those devices are limited in their usefulness in that they provide little information beyond frequency response, and they require time to average the noise signal at the various frequencies measured.
Thence, it can be seen that there has been a need for a method and apparatus for making thorough signal response and distortion measurements of signal transfer devices quickly, so as to take them out of service for a minimal amount of time.